Compression type internal combustion engines require the use of fuel injectors which deliver fuel under pressure to one or more cylinders. Such fuel injectors may be of the solenoid operated type which are operated by an engine control to deliver accurately measured quantities of fuel to the cylinders at precise instants in time based upon the positions of the pistons in the cylinders. The timing of fuel injection and the quantity of fuel injected during each injection operation affect the efficiency of the engine and the emissions therefrom. Thus, it is important to precisely control the timing and quantity of fuel delivered to the cylinders by means of a solenoid driver circuit which accurately controls the fuel injectors.
A prior solenoid driver circuit which is capable of precise timing and fuel quantity control is disclosed in Pflederer U.S. Pat. No. 4,604,675, entitled "Fuel Injection Solenoid Driver Circuit", assigned to the assignee of the present application and the disclosure of which is hereby incorporated by reference. FIG. 1 is a greatly simplified drawing of the Pflederer driver circuit wherein certain elements are identified by the same reference numerals as used in such patent. Each of a series of six fuel injector
solenoid coils 168a-168f is coupled through a modulation switch 164 to a voltage source 10. Cylinder select switches 184a-184f are coupled between the solenoid coils 168a-168f and a series combination of an inductor 186 and a current sensing resistor 188. Flyback diodes 260a-260f include anode terminals which are coupled to the junctions between the coils 168a-168f and the switches 184a-184f. Cathode terminals of the diodes 260a-260f are coupled together to the voltage source 10. During operation of this circuit, an engine control 12 develops command signals which are coupled to cylinder select and current sense/control circuits that in turn operate the switches 184a-184f and a modulation switch 164. When a particular solenoid coil is to be actuated, for example the solenoid coil 168a, the switch 184a is closed by the cylinder select circuit 14. In addition, the current sense/control circuit 16 operates the switch 164 in a pulse width modulated (PWM) mode of operation to control the current delivered to the solenoid coil 168a according to a predetermined control strategy such that power dissipation is kept at a low level.
When the coil 168a has been energized for a sufficient time to insure that the proper quantity of fuel will be delivered to the associated engine cylinder, the switches 184a and 164 are opened, in turn causing flyback currents to flow from ground potential through the parallel combination of a resistor 252 and an inductor 254, a diode 256, the coil 168a and the diode 260a to the voltage source 10. This places a reverse potential across the coil 168a to quickly deenergize same.
While the driver circuit illustrated in the Pflederer patent is effective to control solenoid operated fuel injectors in an efficient manner, it has been found that the driver circuit can be totally disabled under certain. circumstances, in turn leading to a complete shutdown of the engine. Specifically, if either terminal of any of the coils 168a-168f should be shorted to ground potential, there is no way to continue to energize the remaining coils in a controlled manner. Thus, under such a fault condition, there is no way to provide fuel to the cylinders and hence limp-home capability cannot be realized.